Method for producing diesters of terephthalic acid

ABSTRACT

The present invention relates to a process for preparing a terephthalic diester by reacting terephthalic acid with at least one alcohol, wherein terephthalic acid is suspended in the alcohol in a dispersing tank, the preliminary suspension is passed from the dispersing tank into a reactor and converted in the presence of an esterification catalyst to obtain a reaction suspension, a stream of the reaction suspension is drawn off from the reactor, passed through a heat exchanger outside the reactor and heated and the heated reaction suspension is recycled into the reactor, and water of reaction is distilled off together with the vapor as alcohol-water azeotrope, the vapor is at least partly condensed, the condensate is separated into an aqueous phase and an organic phase and the organic phase is at least partly recycled into the reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. § 371)of PCT/EP2015/071579, filed Sep. 21, 2015, which claims benefit ofEuropean Application No. 14186134.4, filed Sep. 24, 2014, both of whichare incorporated herein by reference in their entirety.

The invention relates to a process for preparing terephthalic diestersby reacting terephthalic acid with at least one alcohol.

Esters of terephthalic acid find use as plasticizers and are notable forfavorable toxicological properties.

It is known that carboxylic esters can be prepared by reactingcarboxylic acids with alcohols. This reaction can be conductedautocatalytically or catalytically, for example by means of Brønsted orLewis acids. Irrespective of the manner of catalysis, the result isalways a temperature-dependent equilibrium between the feedstocks(carboxylic acid and alcohol) and the products (ester and water).

In order to shift the equilibrium in favor of the ester (or of the fullester in the case of polybasic acids), an azeotroping agent is generallyused, which helps to remove the water of reaction from the mixture. Ifone of the feedstocks (alcohol or carboxylic acid) has a lower boilingpoint than the ester formed and forms a miscibility gap with water, itis possible to use a reactant as azeotroping agent and recycle it backinto the mixture after water has been removed. In the case ofesterification of higher aliphatic carboxylic acids, aromatic carboxylicacids or di- or polybasic carboxylic acids, the alcohol used isgenerally the azeotroping agent.

If the alcohol used serves as azeotroping agent, the procedure istypically to at least partly condense the vapor from the reactor, toseparate the condensate into an aqueous phase and an organic phaseconsisting essentially of the alcohol used for the esterification, andto recycle the organic phase at least partly into the reactor.

EP-A 1 186 593 describes a process for preparing carboxylic esters byreacting di- or polycarboxylic acids or anhydrides thereof withalcohols, wherein the water of reaction is removed by azeotropicdistillation with the alcohol. The amount of liquid removed from thereaction by the azeotropic distillation is made up again completely orpartly by the alcohol.

WO 2010/076192 A1 proposes removing low boilers from the organic phaseto be recycled in order to prevent the accumulation thereof in thereactor system.

U.S. Pat. No. 7,276,621 B2 describes a process for titanate-catalyzedesterification of terephthalic acid with 2-ethylhexanol. An inert gas ispassed through the reaction mixture in order to promote the removal ofwater.

JP 4956945 B2 also describes a process for esterification ofterephthalic acid with 2-ethylhexanol. In this case, the terephthalicacid is introduced into the reaction system continuously or batchwise asa slurry. The metered addition is effected at the same rate at which theterephthalic acid is converted to the product.

U.S. Pat. No. 7,799,942 B2 describes a process for preparingterephthalic diesters in a reactor at atmospheric pressure using adistillation column atop the reactor. In addition, an inert gas flowsthrough the reaction mixture.

WO 2010/076193 A1 describes a process for purifying the crude esterproduct of an esterification reaction, in which a metallicesterification catalyst is used.

The solubility of terephthalic acid in higher alcohols is low. Forexample, terephthalic acid is soluble in 2-ethylhexanol at 180° C. onlyto an extent of less than 0.65% by weight. The reaction of terephthalicacid with higher alcohols proceeds only via the proportion ofterephthalic acid present dissolved in the alcohol. For the attainmentof high conversions, it is essential to ensure constant mixing of theheterogeneous mixture of terephthalic acid and alcohol, and effectiveintroduction of heat into the reaction system. In addition, it isimportant to keep the water content in the reaction mixture low, inorder to be able to shift the reaction equilibrium to the product sideand, if hydrolysis-sensitive esterification catalysts are used, toprevent the hydrolysis of the catalyst. The metered addition of solidterephthalic acid into the reactor containing boiling alcohol, forexample via a conveying screw, in which the powder drops into thereactor in freefall at the free end of the screw, is possible only withdifficulty because of the risk of the terephthalic acid forming lumps.In the case of tall reactors of high volume, the arrangement of areservoir vessel for terephthalic acid above the reactor is oftenassociated with construction difficulties.

It is therefore an object of the invention to provide a process forpreparing terephthalic diesters which allows simple introduction of theterephthalic acid into the reactor, enables effective mixing of thereaction mixture and achieves effective introduction of heat into thereaction system and full conversion of the terephthalic acid. It is afurther object of the invention to provide a process which can beperformed in existing reactors for esterification reactions throughminor retrofitting.

The object is achieved by a process for preparing a terephthalic diesterby reacting terephthalic acid with at least one alcohol, wherein

-   -   a) terephthalic acid is suspended in the alcohol in a dispersing        tank to obtain a preliminary suspension,    -   b) the preliminary suspension is passed from the dispersing tank        into a reactor and converted in the presence of an        esterification catalyst to obtain a reaction suspension,    -   c) a stream of the reaction suspension is drawn off from the        reactor, preferably from the lowest point in the reactor, passed        through a heat exchanger outside the reactor, preferably counter        to the direction of gravity, and heated and the heated reaction        suspension is recycled into the reactor, and    -   d) water of reaction is distilled off together with the vapor as        alcohol-water azeotrope, the vapor is at least partly condensed,        the condensate is separated into an aqueous phase and an organic        phase and the organic phase is at least partly recycled into the        reactor.

A BRIEF DESCRIPTION OF THE FIGURE

The FIGURE shows a plant suitable for performing the process accordingto the invention.

The process according to the invention can be performed batchwise orcontinuously, but is preferably performed batchwise.

The process gets around the problems associated with the meteredaddition of solid terephthalic acid into the reactor, such as formationof terephthalic acid lumps and blockage of the conveying screw oranother conveying unit. The process provides for the preparation of apreliminary suspension in a dispersing tank. Terephthalic acid ismetered into the reactor not in solid form but in the form of asuspension.

The preliminary suspension is prepared by suspending pulverulentterephthalic acid in a portion of the alcohol in the dispersing tank.For this purpose, a suitable mixing apparatus is used. For instance, anamount of the terephthalic acid can be mixed with alcohol using astirrer; alternatively, dispersing pumps can be used. For example, thetotal amount of terephthalic acid can be suspended in one step, or theterephthalic acid can be suspended in portions over the course of theprocess. For the suspension in portions, it is possible to meterterephthalic acid into the dispersing tank, for example, with the aid ofa conveying screw.

The mixing can also be effected in a closed chamber through theinteraction of a rotating rotor and a stator, in which case only anincremental amount of the components is continuously mixed together ineach case, and the suspension then leaves the chamber.

The alcohol used to prepare the preliminary suspension may be freshalcohol and/or return alcohol, i.e. the organic phase which is obtainedafter condensation of the vapor and phase separation of the condensate.

The dispersing tank usually consists of metallic materials, preferencebeing given to stainless steel. The dispersing tank can be connected tothe reactor on the gas side.

The preliminary suspension is passed into the reactor using a pump or bymeans of gravity. Usable pumps are in principle all the conveying pumpsknown to those skilled in the art that are regarded as suitable in viewof the properties of the preliminary suspension to be conveyed.Conveying pumps usable with preference are a centrifugal pump, pistonpump, screw pump, impeller pump or peristaltic pump. The preliminarysuspension can be metered into the reactor in portions or continuously.The metered addition is preferably effected continuously. Thepreliminary suspension can in principle be metered in at any point inthe reactor, but preference is given to adding the preliminarysuspension in the upper region of the reactor, especially above theliquid level in the reactor. In this way, backflow counter to thedirection of metered addition can very substantially be prevented.

The reactor may be any reactor suitable for performance of chemicalreactions in the liquid phase. Suitable reactors are non-backmixedreactors such as tubular reactors or delay vessels provided withinternals, but preferably backmixed reactors such as stirred tanks, loopreactors, jet loop reactors or jet nozzle reactors. Optionally, it isalso possible to combine a plurality of reactors in a multistageapparatus. Reactors of this kind are, for example, loop reactors withinstalled sieve trays, cascaded vessels, tubular reactors withintermediate feeding or stirred columns.

Preference is given to using a stirred tank reactor. Stirred tankreactors usually consist of metallic materials, preference being givento stainless steel.

Especially preferred is the use of existing reaction systems which areutilized, for example, for the esterification of phthalic anhydride andcan be used for the esterification of terephthalic acid through minorretrofitting. Retrofitting operations are necessary, relatingparticularly to the provision of a dispersing tank.

In the reactor, the preliminary suspension and the esterificationcatalyst are brought into contact, which gives a reaction suspension. Inone embodiment of the process, i) the preliminary suspension is passedinto the unfilled reactor, ii) the preliminary suspension is heated toboiling and iii) the esterification catalyst is added. Optionally, thesequence of steps ii) and iii) can be reversed.

In a preferred embodiment of the process, however, the esterificationcatalyst is initially charged in the reactor in a portion of alcohol,for example 15-50% of the total amount of alcohol, preferably 25-40%.The catalyst/alcohol mixture can first be heated to boiling and then themetered addition of the preliminary suspension can be started.Alternatively, the preliminary suspension is added to thecatalyst/alcohol mixture and then heated. Optionally, the heating of thecatalyst/alcohol mixture and the metered addition of the preliminarysuspension can be performed in parallel.

During the reaction, the reaction suspension in the reactor has atemperature close to the boiling point of the reaction mixture, forexample a temperature of 150° C. to 250° C., preferably of 185° C. to220° C. The boiling point of the reaction suspension is dependent on theratio of terephthalic diester to alcohol and rises over the course ofthe reaction.

According to the invention, heat is introduced into the reaction systemby the passing of the reaction suspension through a heat exchangeroutside the reactor.

This involves drawing off the reaction suspension from the reactor usinga pump and passing it through the heat exchanger. Alternatively, thereaction suspension is conveyed through the heat exchanger by naturalcirculation. Preferably, the reaction suspension is passed through theheat exchanger counter to the direction of gravity. The pump isconnected to the heat exchanger in a fluid-conducting manner. Thereaction suspension is preferably drawn off at the lowest point in thereactor. In this case, the reactor is configured such that the reactionsuspension is drawn off at the geodetically lowest point in the reactor,and there are no dead spaces caused by locally lowest points in thereactor. The pump for drawing off the reaction suspension may inprinciple be disposed at various positions outside the reactor. Forexample, the pump is disposed at the geodetically lowest point in thecircuit consisting of reactor, pump and connecting lines.

Suitable pumps are in principle all the conveying pumps known to thoseskilled in the art that are regarded as suitable for performing theprocess according to the invention in view of the properties of thereaction suspension to be conveyed, usable. Conveying pumps usable withpreference are a centrifugal pump, piston pump, screw pump, impellerpump or peristaltic pump. Very particular preference is given to anaxial or radial centrifugal pump. The heat exchanger is connected to thereactor in a fluid-conducting manner for the recycling of the heatedreaction suspension into the reactor.

Drawing off the reaction suspension at the lowest point in the reactorcan prevent terephthalic acid from settling out. Terephthalic acid isdrawn off from the lower region of the reactor as a suspension inalcohol and passed through the heat exchanger. After being metered backinto the reactor, terephthalic acid is still suspended in alcohol and isavailable for the reaction.

The reaction suspension can in principle be recycled from the heatexchanger into the reactor in principle at any position in the reactor,but it is appropriately recycled in the upper region of the reactor, forexample at the height of the liquid level of the reaction suspension orin the range from the height of the liquid level of the reactorsuspension to 30% below it. The volume flow rate which is conductedthrough the heating apparatus is chosen, for example, such that thecomplete reactor contents are circulated within a period of 1 to 60minutes, preferably 1 to 10 minutes. The constant circulation of thereactor contents assures effective mixing of the reaction suspension.

Preferably, the stream of the reaction suspension is run through theheat exchanger outside the reactor counter to the direction of gravity,i.e. from the bottom upward. The specified direction of the streamcounter to gravity prevents sedimentation of terephthalic acid in theheat exchanger.

The reaction suspension is heated by the passage through a heatexchanger to a temperature at which a sufficiently large vapor flow ratearises at the surface of the reaction mixture to discharge the water ofreaction, for example to a temperature of 150 to 250° C., preferably 180to 220° C.

Optionally, the mixing of the reaction suspension can be promoted by themetered addition of an inert gas into the reactor, especially at thelowest point in the reactor, and/or the stream of the reactionsuspension. Especially in the event of disrupted operation of the pumpfor the drawing-off of the reaction suspension, for example in the eventof failure of the pump, the metered addition of the inert gascontributes to preventing sedimentation of terephthalic acid at the baseof the reactor and/or in pipelines. Preferably, the inert gas is meteredin on the suction side of the pump. Alternatively, the metered additioncan be effected on the pressure side of the pump. This enablesmaintenance of the circulation through the heat exchanger. Inert gasesare all gases which do not have any reactivity with the constituents ofthe reaction suspension under the reaction conditions, especiallynitrogen or argon. Preferably, the inert gas is metered in an amount of0.01 to 5 units by volume of the inert gas per unit by volume of thereaction suspension per hour.

During the reaction, an alcohol-water azeotrope is distilled offtogether with the vapor, the vapor is at least partly condensed, thecondensate is separated into an aqueous phase and an organic phase andthe organic phase is at least partly recycled into the reactor.

Condensation or partial condensation of the vapor can be effected usingany suitable condensers. These can be cooled with any desired coolingmedia. Condensers with air cooling and/or water cooling are preferred,and air cooling is particularly preferred.

The condensate obtained is subjected to a phase separation into anaqueous phase and an organic phase. For this purpose, the condensate istypically passed into a phase separator (decanter), where it dividesinto two phases as a result of mechanical settling, and these can bedrawn off separately. The aqueous phase is removed and, optionally afterworkup, can be discarded or used as stripping water in theaftertreatment of the ester.

The organic phase is preferably recycled into the reactor through acolumn (called return alcohol column) in which the recycled organicphase is run counter to at least a portion of the vapor. The returnalcohol column may, for example, be a tray column, column withstructured packing or column with random packing. A small number ofplates is generally sufficient. A suitable example is a column having 2to 10 theoretical plates. Preferably, the column is placed atop thereactor, i.e. connected directly to the reactor. Appropriately, theorganic phase is introduced into the return alcohol column at the top orin the upper region. The condensate running off from the return alcoholcolumn passes back into the reactor. The recycling of the organic phasevia the return alcohol column has the advantage that the recycledorganic phase is preheated and freed of traces of water which haveremained in the organic phase after the phase separation or aredissolved in the organic phase in accordance with their thermodynamicsolubility. The water content in the recycled organic phase is less thanthe maximum solubility of water in the alcohol, preferably less than 3%by weight, especially less than 0.5% by weight.

In the process according to the invention, preference is given to usinglinear, branched or cyclic aliphatic alcohols having 4 to 18 carbonatoms, especially 8 to 14 carbon atoms, or aromatic alcohols. Thealcohols are monools and/or polyols and may be tertiary, secondary orprimary.

The alcohols used may originate from various sources. Suitablefeedstocks are, for example, fatty alcohols, alcohols from the Alfolprocess, or alcohols or alcohol mixtures which have been obtained byhydrogenating saturated or unsaturated aldehydes, especially those whosesynthesis includes a hydroformylation step.

Aliphatic alcohols which are used in the process according to theinvention are, for example, n-butanol, isobutanol, pentanols, hexanols,heptanols, octanols such as n-octanol, 2-ethylhexanol, nonanols, decylalcohols or tridecanols, prepared by hydroformylation or aldolcondensation and subsequent hydrogenation. The alcohols can be used as apure compound, as a mixture of isomeric compounds or as a mixture ofcompounds having different numbers of carbon atoms. One example of suchan alcohol mixture is a C₉/C₁₁ alcohol mixture.

Aromatic alcohols which can be used in the process according to theinvention are, for example, phenol, benzyl alcohol, 1-naphthol,2-naphthol, 1,2-dihydroxybenzene, 1,3-dihydroxybenzene,1,4-dihydroxybenzene, 1,4-naphthohydroquinone, 2,4,6-trinitrophenol,primary phenylethyl alcohol, secondary phenylethyl alcohol, phenylpropylalcohol, o-tolyl alcohol, p-tolyl alcohol, cuminic alcohol,p-nitrophenol, m-, o- or p-alkylphenol, e.g. m-, o- or p-methylphenol orm-, o- or p-ethylphenol, m-, o- or p-halophenol, e.g. m-, o- orp-chlorophenol or m-, o- or p-bromophenol. In addition, it is possibleto use p-nitrobenzyl alcohol, m-, o- or p-alkylbenzyl alcohol, e.g. m-,o- or p-methylbenzyl alcohol or m-, o- or p-ethylbenzyl alcohol, m-, o-or p-halobenzyl alcohol, e.g. m-, o- or p-chlorobenzyl alcohol or m-, o-or p-bromobenzyl alcohol, 2-ethoxyphenol, 3-ethoxyphenol,4-ethoxyphenol, 2-propoxyphenol, 3-propoxyphenol, 4-propoxyphenol,2-ethoxybenzyl alcohol, 3-ethoxybenzyl alcohol, 4-ethoxybenzyl alcohol,2-propoxybenzyl alcohol, 3-propoxybenzyl alcohol or 4-propoxybenzylalcohol.

Polyols which can be used in the process according to the invention are,for example, propane-1,2-diol, propane-1,3-diol, butane-1,2-diol,butane-1,3-diol, butane-1,4-diol, neopentyl glycol, pentane-1,5-diol,hexane-1,6-diol, decane-1,10-diol, diethylene glycol,2,2,4-trimethylpentane-1,5-diol, 2,2-dimethylpropane-1,3-diol,1,4-dimethylolcyclohexane, 1,6-dimethylolcyclohexane, glycerol,trimethylolpropane, erythritol, pentaerythritol and sorbitol.

Particularly preferred alcohols are 2-ethylhexanol, 2-propylheptanol,isononanol isomer mixtures, decanol isomer mixtures and C₉/C₁₁ alcoholmixtures.

The alcohol to be converted, which serves as azeotroping agent, can beused in a stoichiometric excess. Preferably, the amount of alcohol usedis selected such that 10% to 35% by weight of alcohol is present in thecrude product of the reaction, based on the theoretical full conversionof the terephthalic acid.

The inventive esterification is conducted in the presence of anesterification catalyst.

In a preferred embodiment of the process according to the invention, theesterification catalyst is soluble in the alcohol.

The esterification catalyst is suitably selected from Lewis acids suchas alkoxides, carboxylates and chelate compounds of titanium, zirconium,hafnium, tin, aluminum and zinc; boron trifluoride, boron trifluorideetherates; mineral acids such as sulfuric acid, phosphoric acid;sulfonic acids such as methanesulfonic acid and toluenesulfonic acid,and ionic fluids.

Suitably, the esterification catalyst is selected from alkoxides,carboxylates and chelate compounds of titanium, zirconium, hafnium, tin,aluminum and zinc. Suitable substances include tetraalkyl titanates suchas tetramethyl titanate, tetraethyl titanate, tetra-n-propyl titanate,tetraisopropyl titanate, tetra-n-butyl titanate, tetraisobutyl titanate,tetra-sec-butyl titanate, tetraoctyl titanate, tetra(2-ethylhexyl)titanate; dialkyl titanates ((RO)₂TiO in which R is, for example,isopropyl, n-butyl, isobutyl) such as isopropyl n-butyl titanate;titanium acetylacetonate chelates, such asdiisopropoxybis(acetylacetonate)titanate,diisopropoxybis(ethylacetylacetonate)titanate,di-n-butylbis(acetylacetonate)titanate,di-n-butylbis(ethylacetoacetate)titanate,triisopropoxybis(acetylacetonate)titanate; zirconium tetraalkoxides suchas zirconium tetraethoxide, zirconium tetrabutoxide, zirconiumtetrabutyrate, zirconium tetrapropoxide, zirconium carboxylates such aszirconium diacetate; zirconium acetylacetonate chelates such aszirconium tetra(acetylacetonate), tributoxyzirconium acetylacetonate,dibutoxyzirconium bis(acetylacetonate); aluminum trisalkoxides such asaluminum triisopropoxide, aluminum trisbutoxide; aluminumacetylacetonate chelates such as aluminum tris(acetylacetonate) andaluminum tris(ethylacetylacetonate). More particularly, isopropyln-butyl titanate, tetra(isopropyl) orthotitanate or tetra(butyl)orthotitanate or mixtures thereof are used.

Suitable ionic fluids (ionic liquids) are, for example,methylimidazoliumbutanesulfonic acid triflate and1-ethyl-3-methylimidazolium hydrogensulfate.

The catalyst concentration depends on the type of catalyst. In thetitanium compounds used with preference, it is 0.001 to 1.0 mol % basedon the amount of terephthalic acid, especially from 0.01 to 0.2 mol %.

The reaction temperatures are between 150° C. and 250° C. The optimaltemperatures depend on the feedstocks, progress of the reaction andcatalyst concentration. They can be determined easily by experiments foreach individual case. Higher temperatures increase the reaction ratesand promote side reactions, for example olefin formation or formation ofcolored by-products. It is necessary for removal of the water ofreaction that the alcohol can be distilled out of the reaction mixture.The desired temperature or the desired temperature range can beestablished via the pressure in the reactor. In the case of low-boilingalcohols, therefore, the reaction can be performed at elevated pressure,and in the case of higher-boiling alcohols under reduced pressure. Forexample, in the reaction of terephthalic acid with 2-ethylhexanol, atemperature range from 180° C. to 220° C. is employed within thepressure range from 300 mbar to 2 bar.

Appropriately, the reactor and dispersing tank will be will be operatedat essentially the same pressure, especially about ambient pressure.Optionally, the reactor and dispersing tank can also be operated atdifferent pressures.

Preference is given to performing the process according to the inventionuntil the terephthalic acid has been essentially fully converted. Theconversion can be determined via the determination of the acid number ofthe reaction suspension. The acid number is determined by neutralizing asample of the reaction suspension with tetrabutylammonium hydroxide. Themass of tetrabutylammonium hydroxide consumed in the neutralization canbe used to determine the molar amount of tetrabutylammonium hydroxideconsumed, and stoichiometric considerations to determine the molaramount of free acid groups in unconverted terephthalic acid. Proceedingfrom the known molar amount of terephthalic acid used, it is thuspossible to determine the conversion. Additional methods for determiningthe conversion are HPLC analyses and the measurement of the turbidity ofthe reaction suspension by inline turbidity measurements. In the processaccording to the invention, a conversion greater than 99% is preferablyachieved.

After the reaction has ended, the reaction mixture consistingessentially of the desired ester and excess alcohol comprises, as wellas the catalyst and/or conversion products thereof, small amounts ofester carboxylic acid(s) and/or unconverted carboxylic acid.

These crude ester mixtures are worked up by admixing the crudedi(C₄-C₁₈-alkyl) terephthalate with an aqueous base, evaporating waterout of the mixture obtained, admixing the liquid phase obtained withwater to form a water-in-oil emulsion, distilling water out of theemulsion and filtering the di(C₄-C₁₈-alkyl) terephthalate.

First of all, the esterification catalyst is deactivated andprecipitated by adding an aqueous base. At the same time, the acidand/or partial ester of the acid unconverted in the esterificationreaction are converted to salts.

The aqueous base can be added in any suitable manner. It is preferablyadded beneath the liquid surface of the crude ester. Suitable apparatusfor this purpose include, for example, probes and nozzles provided atone end of the vessel or the vessel wall. The mixture is then mixedvigorously, for example by means of a stirrer or circulation pump.

The amount of aqueous base added should be such that it is sufficientfor complete neutralization of the acidic components of the crude ester.In practice, a greater or lesser excess of base is used. The totalamount of the acidic components of the crude ester is appropriatelydetected via the acid number (in mg KOH/g). Preference is given tointroducing 100% to 300% neutralization equivalents with the aqueousbase, based on the acid number of the crude ester, especially 130% to220%. A neutralization equivalent is understood to mean the amount ofbase that can bind the same number of protons as 1 mg of KOH. In otherwords, an excess of base of up to 200% is used, preferably 30% to 120%.

Useful aqueous bases include solutions of hydroxides, carbonates,hydrogencarbonates of alkali metals and alkaline earth metals. Aqueousalkali metal hydroxide solutions are generally preferred. Aqueous sodiumhydroxide solution is particularly preferred because of its ease ofavailability.

The concentration of the aqueous base is not critical per se, but theremay be hydrolysis of the esters at the site of introduction of the basewhen concentrated alkali solutions are used. On the other hand, theconcentration of the aqueous base should not be too low, since the waterintroduced with the aqueous base has to be removed again in thesubsequent step. Therefore, preference is given to aqueous bases ofmoderate to low concentration, for example those of a concentration of0.5% to 25% by weight, especially 1% to 10% by weight. Aqueous sodiumhydroxide solution having a concentration of 1% to 5% by weight isparticularly preferred.

Often, the precipitated solid consisting essentially of catalystbreakdown products and salts of unconverted acid or partial esters ofpolybasic acids is present in finely divided form and is difficult tofilter. Appropriately, the fine particles are agglomerated to larger,readily removable particles.

For this purpose, the liquid phase is admixed with water to form awater-in-oil emulsion. The water is distributed as a disperse phase inthe form of fine droplets in the liquid organic phase. The fine solidparticles migrate to the interface between water droplets andsurrounding organic phase. In the course of the subsequent evaporationof the water, the fine particles agglomerate and form coarse,efficiently removable particles.

In order that a separate water phase forms, the amount of water addedmust be greater than that corresponding to the solubility of water inthe organic phase. One factor on which the water solubility in theorganic phase depends is the content of unconverted alcohol, since thealcohol acts as a solubilizer. The higher the alcohol content, the morewater has to be added to form an emulsion. In the case of typicalresidual alcohol contents of 20% to 30% by weight, suitable amounts aregenerally from 10 to 80 g of water, preferably 30 to 60 g, based on 1 kgof crude ester.

The water phase is divided into fine droplets with a suitable stirrer orhomogenizer, or by pumped circulation of the emulsion using acirculation pump. The water droplets produced preferably have a meandroplet size of less than 1000 μm. Examples of suitable stirrers havinga high specific stirrer input are disk stirrers. Alternatively,particularly in the case of a continuous process regime, it is possibleto use a mixing nozzle in which water is added directly to the crudeester stream via a dispersing valve.

The emulsion is appropriately formed at about standard pressure.

The water in the emulsion thus produced is distilled off again in thenext step.

After this treatment, the solids are in efficiently filterable form; nofines fraction passes through in the filtration. Suitable filters forfiltration of the ester are all suitable filters such as chamber filterpresses, belt filters, cartridge filters or pan filters. For acontinuous process regime, pan filters with centrifugal cake ejectionare particularly suitable. The solids removed are discarded.

After the filtration, the ester can be subjected to variousaftertreatments, such as a steam stripping or the like.

The invention is illustrated in detail by the appended FIGURE.

FIGURE shows a plant suitable for performing the process according tothe invention.

According to FIGURE, alcohol from the reservoir 9 and terephthalic acidfrom the reservoir 10 are metered into a dispersing tank 7 and mixed toform a preliminary suspension using a stirrer 11. The preliminarysuspension is passed into the upper region into the reactor 1 with theaid of a pump 8. Within the reactor 1 are a further portion of thealcohol and the esterification catalyst. Preferably at the lowest pointin the reactor 1, the reaction suspension is drawn off from the reactor1 using a pump 2 outside the reactor and conducted through a heatexchanger 3 outside the reactor. The reaction suspension heated in theheat exchanger 3 is recycled back into the reactor 1 in the upper regionthereof. The vapor passes through the column 6 and is at least partlycondensed in the condenser 4. In the phase separator 5, the condensateis separated into an aqueous phase and an organic phase. The aqueousphase is discarded; the organic phase is recycled into the reactor viacolumn 6.

The invention claimed is:
 1. A process for preparing a terephthalicdiester which comprises reacting terephthalic acid with at least onealcohol, wherein a) suspending terephthalic acid in the alcohol in adispersing tank to obtain a preliminary suspension, b) passing thepreliminary suspension from the dispersing tank into a reactor andconverted in the presence of an esterification catalyst to obtain areaction suspension, c) drawing off a stream of the reaction suspensionfrom the reactor, passed through a heat exchanger outside the reactorand heated and the heated reaction suspension is recycled into thereactor, and d) distilling off water of reaction together with the vaporas alcohol-water azeotrope, the vapor is at least partly condensed, thecondensate is separated into an aqueous phase and an organic phase andthe organic phase is at least partly recycled into the reactor.
 2. Theprocess according to claim 1, wherein the stream of the reactionsuspension is drawn off from the lowest point in the reactor.
 3. Theprocess according to claim 1, wherein the organic phase, before beingrecycled into the reactor, is conducted through a column in which theorganic phase is run counter to at least a portion of the vapor.
 4. Theprocess according to claim 1, wherein the organic phase recycled intothe reactor has a water content lower than the solubility of water inthe alcohol.
 5. The process according to claim 1, wherein the organicphase recycled into the reactor has a water content of less than 3% byweight.
 6. The process according to claim 1, wherein the esterificationcatalyst is a Lewis acid, mineral acid, sulfonic acid or an ionic fluid.7. The process according to claim 1, wherein the esterification catalystis alkoxide, carboxylate or chelate compounds of titanium, zirconium,hafnium, tin, aluminum or zinc; boron trifluoride; boron trifluorideetherates; sulfuric acid; phosphoric acid; methanesulfonic acid ortoluenesulfonic acid.
 8. The process according to claim 1, wherein theesterification catalyst is acidic ion exchangers, zeolites, oxidesand/or hydroxides of magnesium, aluminum, zinc, titanium, silicon, tin,lead, antimony, bismuth, molybdenum or manganese.
 9. The processaccording to claim 1, wherein the esterification catalyst is soluble inthe alcohol.
 10. The process according to claim 1, wherein the alcoholis a linear aliphatic C₄-C₁₈ alcohol, a branched aliphatic C₄-C₁₈alcohol, cyclic aliphatic C₄-C₁₈ alcohol or an aromatic alcohol.
 11. Theprocess according to claim 1, wherein the alcohol is used in such astoichiometric excess that the crude esterification product comprises15% to 35% by weight of alcohol.
 12. The process according to claim 1,wherein an inert gas is metered into the reactor and/or the stream ofthe reaction suspension for fluidization.
 13. The process according toclaim 12, wherein the inert gas is metered in in an amount of 0.1 to 5units by volume of the inert gas per unit volume of the reactionsuspension per hour.
 14. The process according to claim 1, wherein thecrude terephthalic diester is worked up by admixing with an aqueousbase, evaporating water out of the mixture obtained, admixing the liquidphase obtained with water to form a water-in-oil emulsion, distillingwater out of the emulsion and filtering the terephthalic diester. 15.The process according to claim 1, wherein the reaction suspension ispassed through the heat exchanger at a volume flow rate chosen such thatthe reactor contents are circulated completely within a period of 1 to10 minutes.